During bacterial chemotaxis in Escherichia coli, adaptation is accomplished by reversible methylation of the transmembrane signal transducers. Methyl groups are added by the CheR protein in a slow response to attractants and removed by the CheB protein in response to repellents. The methylesterase activity of the CheB protein is modulated by a factor that is controlled in a global fashion throughout the cell. By controlling the level of expression of the cheR, cheB, and transducer genes with exogenous promoters on multicopy plasmids, we demonstrate that the modulating factor exists in stoichiometric concentrations relative to CheB protein and that the generation or efficacy of this factor requires the cheA and/or cheW gene products, suggesting that phosphorylation of the methylesterase by CheA may be involved in its global activation. We show that in the absence of any modulation of the CheB activity, the CheR methyltransferase activity is modulated in a local fashion at the transducers, most likely as a result of a conformational change in the transducer protein brought about by the binding of ligand, and does not require CheA or CheW
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